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7 Structure of Rock Bodies Structure of 7 Rock Bodies Although many people think that Earth’s crust is permanent and fixed, evidence of crustal movement comes in many forms and is there for all to see. In the Mediterranean area, some ancient harbors, such as Ephesus in Asia Minor, are now high and dry, several kilometers inland from the sea. Other ancient shorelines have been submerged well below low tide. Earthquakes are perhaps the most convincing evidence that the crust is moving. During earthquakes, the crust not only vibrates, but segments of it are fractured and displaced along a fault. One impres- sive example was the movement along the San Andreas Fault during the 1906 San Francisco earthquake, which offset fences and roads by as much as 7 m.Another is the 1899 earthquake at Yakutat Bay,Alaska, during which a beach was uplifted 15 m above sea level.The raised beach terraces along the coast of southern California also testify to crustal movement in prehistoric times.There, ancient wave-cut cliffs and terraces—containing remnants of beaches with barna- cles, shells, and sand—rise in a series of steps more than 500 m above the present shore. Since the beginning of geologic studies more than 200 years ago, geologists have shown that rock layers in certain parts of the continents are folded, fractured, and deformed on a 166 gigantic scale. The large-scale shape, geometry, and deformation of rock bodies are part of what we call their structure. Deformation of the crust is most intense in the great mountain belts of the world, where sedimentary rocks, which were originally horizontal and below sea level, are now folded, contorted, fractured, and, in some places, completely overturned. In some mountains, large bodies of rock have been thrust several tens of kilometers over younger strata.The folded rocks in the world’s major mountain belts (the Appalachians, the Rockies, the Andes, the Himalayas, the Urals, and the Alps) all exemplify this type of deformation. These folded and warped rock layers testify to the continuing motion of the lithosphere and the defor- mation it produces. Spectacular examples of structural deformation are found in the large flexures of folded mountain belts. In the photo above of Borah Peak, Idaho, the sedimentary rock layers were originally deposited on the seafloor, but were uplifted more than four kilometers above sea level and deformed into anticlines and synclines by horizontal compression at a convergent plate margin. Erosion has cut through the flexures to reveal their internal structure. The extensive deformation of rock bodies in the shields and mountain belts shows that Earth’s tectonic system has operated throughout geologic time, with shifting plates con- stantly deforming at their margins. In this chapter we examine how and why rocks deform, how we describe deformed rocks, and then give specific examples of deformed rock bodies. 167 MAJOR CONCEPTS 1. Deformation of Earth’s crust is well documented in historical times by earth- quakes along faults, by raised beach terraces, and by deformed rock bodies. 2. Rocks deform when applied stress exceeds their strength.They may deform by ductile flow or brittle fracture. Extensional stress causes rocks to stretch and thin. Contractional stress causes rocks to shorten and thicken. 3. Joints are fractures in rocks along which there is no horizontal or vertical displacement. 4. Faults are fractures, along which slippage or displacement has occurred. The three basic types are (a) normal faults, (b) thrust faults, and (c) strike-slip faults. 5. Folds in rock strata range in size from microscopic wrinkles to large structures hundreds of kilometers long. The major types of folds are (a) domes and basins, (b) plunging anticlines and synclines, and (c) complex folds. PRINCIPLES OF ROCK DEFORMATION Rocks deform in response to differential stress. The resulting structure depends on the stress orientation. At high temperatures, ductile flow of rocks occurs. At low temperatures, brittle fractures form. The folds and faults exposed in canyon walls and mountain ranges show that crustal rocks can be deformed on large scales and in dramatic ways (Figure 7.1). But why are some rocks warped into great folds and others only fractured or faulted? Why are some only gently folded, whereas others are complexly folded and faulted? In short, what factors control the type of deformation that rocks experience? To understand this, you need to understand the forces on rocks. Force applied to an area is stress. Stress is the same thing as pressure and is a measure of the in- tensity of the force or of how concentrated the force is. Everyday experience tells us that solids will bend or break if too much stress is placed on them; that is, they deform if the stress exceeds their strength (their natural resistance to deforma- tion). Rocks behave in the same way and deform in response to the forces applied to them. All of Earth’s rocks are under some type of stress, but in many situations the stress is equal in all directions and the rocks are not deformed. In many tectonic How are solid rocks deformed? settings, however, the magnitude of stress is not the same in all directions and rocks experience differential stress. As a result, the rocks yield to the unequal stress and deform by changing shape or position. Geologists call the change in shape strain. In other words, differential stress causes strain. Although strain proceeds by several complex phenomena, two end-member styles of deformation are recognized, with all gradations between them being pos- sible. Under some conditions, rock bodies change shape by breaking to form con- tinuous fractures and they lose cohesion; this is brittle deformation (Figure 7.2). We commonly see such behavior of solids in our daily experience: Chairs, baseball bats, pencils, and wooden beams break if too much force is applied to them. On the other hand, ductile deformation occurs when a rock body deforms permanently without fracturing or losing cohesion. The most obvious type of ductile deforma- tion is the viscous flow of fluids, such as molten magma, but solids can also deform ductilely.At first it may seem strange that solids can flow. But bending of metal pro- vides a familiar example. Consider sheet metal in a car fender. A minor collision commonly makes a dent, not a fracture in the fender. Likewise, rocks can flow in a solid state, under the right conditions. This type of solid state flow is usually called plastic flow and is accomplished by slow internal creep, gliding on imperfections in crystals, and recrystallization. Depending on the temperature or pressure of the surroundings and the rate at which stress is applied, most types of rocks can deform by brittle fracture or 168 Structure of Rock Bodies 169 Removed by erosion FIGURE 7.1 A sequence of inclined beds striking toward the background and dipping 40° to the east (left) forms the flank of the San Rafael Swell, in Utah. The diagram shows the form of the flexure and the upper beds, which have been partially removed by erosion. ductile flow. Low pressures, low temperatures, and rapid deformation rates favor brittle deformation (Figure 7.2B).As a result, brittle structures are most common in the shallow crust. We use the term shear to describe slippage of one block past another on a fracture. High confining pressures, high temperatures, and low rates of deformation all favor ductile behavior (Figure 7.2C). Ductile deformation is more common in the mantle and deeper parts of the crust.Another example of this difference can be seen in the behavior of glass.When a glass rod is cold, it is strong and brittle fractures form when enough stress is applied (Figure 7.3). When the What effect does confining pressure same glass rod is hot but not molten, it is weak and easy to bend. To visualize the have on the style of rock deformation? role of the rate of deformation, consider taffy (or Silly Putty™) as an example. If warm taffy is pulled slowly and steadily, it is ductile and stretches continuously, without breaking, to form long, thin strands. On the other hand, if it is stretched rapidly, it may break and form brittle fractures. It should be clear which types of rock structures form by ductile deformation and which form by brittle behavior.The flow of rocks in a solid state to form folds in metamorphic rocks is a good example of ductile behavior (Chapter 6 Opener). The formation of fractures, joints, and faults are common expressions of defor- mation in the brittle regime. (A) Initial shape (B) Low confining pressure (C) High confining pressure FIGURE 7.2 Brittle versus ductile behavior of rocks is controlled by external conditions. These marble cylinders were compressed in a laboratory under different confining pressures, but at the same temperature and with the same deforming stress. At low pressure, the cylinder deformed in a brittle fashion and fractured and faulted. At high pressure, the cylinder deformed in a ductile fashion as mineral grains flowed and recrystallized. 170 Chapter 7 FIGURE 7.3 Temperature affects how solids deform. (Photographs by Stan (A) A cold glass rod deforms by brittle fracture (B) A hot glass rod bends and shows ductile Macbean) and breaks. behavior without breaking. We can also better understand rock structures if we consider the orientations of the stresses acting on a body (Figure 7.4). For example, tension occurs where the stresses point away from one another and tend to pull the rock body apart. In con- trast, compression tends to press a body of rocks together. Three distinct types of deformation occur because of differential stresses caused by tectonic processes (Figure 7.4).To visualize this, imagine how two adjacent blocks can interact.They can move away from one another (extension), move toward one another (con- traction), or slip horizontally past one another (lateral-slip).
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